The present invention relates to fuel cells, particularly to small, compact fuel cells, and more particularly to a miniature power source composed of a stack of fuel cells fabricated by combining MEMS and thin film deposition technologies to produce fuel cells with microflow channels, fully-integrated control circuitry, and integrated resistive heaters.
Portable power sources of various types have been under development for many years. A serious need exists for portable power sources with significantly higher power density, longer operating lifetime, and lower cost. Present rechargeable and primary portable power sources have excessive weight, size, and cost with limited mission duration. As an example, batteries covering power range from 1-200 Watts have specific energies ranging from 50-250 Whr/Kg, which represents two to three hours of operation for a variety of commercial and military applications. An alternative power source is the fuel cell which would potentially provide higher performance power sources for portable power applications if the stack structure, packaging, and cell operation were made compatible with scaling down of size and weight.
Fuel cells typically consist of electrolyte materials based on either polymer (proton exchange type) or solid oxide materials, which are sandwiched between electrodes. The fuel cell operates when fuel (usually hydrogen) is delivered to one electrode, and oxygen to the other. By heating the electrode-electrolyte structure, the fuel and oxidant diffuse to the electrode interfaces where an electrochemical reaction occurs, thereby releasing free electrons and ions which conduct across the electrolyte. Typical fuel cells are made from bulk electrode-electrolyte materials which are stacked and manifolded using stainless steel or other packaging which is difficult to miniaturize. These systems are bulky, requiring labor intensive manual assembly, packaging and testing, and in the case of solid oxide materials, typically operate at high temperatures ( greater than 600xc2x0 C.). If the electrode-electrolyte stack can be made very thin and deposited using thin film deposition techniques, the temperature of operation will be significantly lower, and the cost of integration, packaging and manufacturing can be reduced.
Previous efforts at Lawrence Livermore National Laboratory, for example, have demonstrated the synthesis of a thin-film solid-oxide based electrolyte fuel cell. See A. F. Jankowski et al., Mat Res. Soc. Symp. Proc., Vol. 496, pp 155-158, 1998 Material Research Society; and U.S. Pat. No. 5,753,385, issued May 19, 1998 to A. F. Jankowski. In one example, the thin film solid oxide fuel cell (TFSOFC) stack was formed using physical vapor deposition (PVD) techniques. The host substrate used was a silicon wafer covered by a thin layer of silicon nitride. A layer of nickel was first deposited, followed by a layer yttria-stabilized zirconia (YSZ). The conditions during the deposition were adjusted in order to achieve smooth, dense, continuous films, thus, avoiding pinhole formation which could result in electrical shorting through the electrolyte layer. This enables the electrolyte layer to be on the order of 1 xcexcm thick rather than typical thicknesses on the order of  greater than 10 xcexcm for bulk solid oxide fuel cells. By thinning the electrolyte layer, the diffusive path for the oxygen ion is shorter and the fuel cell operates at much lower temperatures. A silver electrode layer is deposited on top of the YSZ layer. The deposition conditions of this film are adjusted to create a porous structure so that oxygen can readily diffuse to the electrolyte interface.
The present invention combines an example of thin-film deposition technology, referenced above, with micro-electro-mechanical systems (MEMS) technology to produce a thin-film miniature fuel cell with microflow channels and full-integrated control circuitry, along with integrated resistive heaters for effectively heating the fuel cell such that it will yield and order of magnitude greater power density than any currently known fuel cell. Using this combined technology, thin-film fuel cell stacks can be produced to provide a small, compact miniature power source. The miniature fuel cells of this invention may be either solid oxide or solid polymer or proton exchange membrane electrolyte materials, and may also utilize catalyst layers between the electrodes and the electrolyte.
It is an object of the present invention to provide a small, compact fuel cell power source.
A further object of the invention is to provide thin-film fuel cells for electrical power applications.
A further object of the invention is to provide MEMS-based thin-film fuel cells.
Another object of the invention is to provide an MEMS-based thin-film fuel cells having microflow channels and integrated resistive heaters and control circuitry.
Another object of the invention is to provide a MEMS-based thin-film fuel cell stack, manifold structure, and fuel reservoir as a miniature portable power source.
Another object of the invention is to provide a MEMS-based thin-film fuel cell capable of utilizing an electrolyte of either a solid oxide, a solid polymer, or a proton exchange membrane material.
Another object of the invention is to provide thin-film fuel cell stack with microflow channels, full-integrated control circuitry, and integrated resistive heaters capable of producing a high power density.
Another object of the invention is to provide a MEMS-based fuel cell which includes electrode catalyst/electrolyte materials which enable the combination of a fuel and oxidant above room temperature to produce continuous electric current.
Another object of the invention is to provide a MEMS-based fuel cell having in the form of a module which can be directly stacked as a means of scaling the power and voltage.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically the invention involves combining MEMS technology and thin-film deposition technology to produce a miniature thin-film fuel cell or fuel cell stack as a portable power source. The MEMS-based fuel cell of this invention may utilize an electrolyte layer which may consist of either a solid oxide, a solid polymer, or a proton exchange membrane material, and may utilize catalyst layers between the electrodes and the electrolyte. The fuel cell includes microflow channels and manifolding micromachined into the host structure/substrate, and may contain a rechargeable microbattery for startup control, and utilize integrated resistive heaters for efficient heating of the stack materials, and may utilize integrated microvalves, resistive heaters, or other means to control the flow of fuel to the fuel cell stack. Furthermore, the fuel cell may exploit the various fuel cell electrode/electrolyte/catalyst materials systems as they are developed such that either direct hydrogen fuel is used, or other liquid hydrocarbon fuels, including methanol or propanol, can be used. Additionally, other structures or materials may be readily integrated with a MEMS-based fuel cell which enable reforming of fuel such that a variety of fuels having high percentage concentrations of hydrogen may be used. The MEMS-based fuel cell may incorporate a fuel reservoir as part of a package approach, or as a modular cartridge which can be easily replaced or recharged. Such a fuel reservoir may simply be a volume containing a liquid if liquid fuels are used, or a volume containing a metal hydride or other material which is capable of storing hydrogen within it. In this case, some form of valve may be placed in the micro-flow channels as a means of controlling the flow of fuel to the stack. Additionally, heating elements may be used to control the flow of hydrogen from the reservoir. The MEMS-based fuel cell modules may be directly stacked on top of each other as a means of scaling voltage and power, or connect fuel cell mo on a planar level to do the same. Power transforming circuitry utilized to provide the appropriate output voltage and power